1. Field of the Invention
The invention relates to a free-piston engine.
2. Description of the Related Art
A free-piston engine fundamentally is a combustion engine working according to the 2-cycle method and having not a crankshaft drive but a hydraulic circuit including a reciprocating pump as its subsequently arranged drive train. To this end, the engine piston is connected to a hydraulic cylinder whereby the translatory energy generated during a work cycle of the engine is supplied directly to the hydraulic work medium, without the classical by-way of the rotary movement of a crankshaft drive. The subsequently arranged, storage-capability hydraulic circuit is designed such as to absorb the output power and buffer it for supplying it to a hydraulic output unit, e.g., an axial piston engine, in accordance with power demand.
In DE 40 24 591 A1 a free-piston engine of the generic type is described, also known as a Brandl free-piston engine. In the case of this concept, the compression movement of the engine piston takes place through co-operation with a hydraulic piston which may be connected to a high-pressure accumulator or a low-pressure accumulator via a 2/3-way switchover valve. At the beginning of the compression stroke, an acceleration of the engine piston takes place through applying the pressure in the high-pressure accumulator to the hydraulic cylinder. Once a predetermined engine piston velocity is reached, the hydraulic cylinder is connected to the low-pressure accumulator via the switchover valve, so that the further compression stroke of the engine piston takes place against the effective force from the compression pressure of the work gas. After the outer dead center (AT) has been reached, the work gas is ignited, and the engine piston is accelerated towards the inner dead center (IT). During this piston movement from AT to IT, the connection with the high-pressure accumulator is controlled open via the switchover valve, whereby the engine piston is decelerated and the kinetic energy thereof is converted to potential hydraulic energy, and the high-pressure accumulator is charged. Although the response times of the switchover valve are in the milliseconds range, throttling losses possibly in the order of 10% of the engine power are engendered in the switchover valve by controlling the connection to the high-pressure accumulator open and closed.
These drawbacks of the Brandl free-piston engine may be overcome with the aid of another free-piston design, the so-called INNAS engine as disclosed, e.g., in WO 9603576 A1.
In this INNAS free-piston engine the hydraulic piston is designed as a step piston and has two effective surfaces, the larger, first one of which is arranged in a compression cylinder, while the smaller, second one defines a pump working chamber or work cylinder. The large surface is capable of being subjected to the pressure in a compression cylinder, whereas the work cylinder may be connected with a high-pressure accumulator or a low-pressure accumulator via check valves. This INNAS free-piston engine has a substantially more complex structure in comparison with the Brandl free-piston engine, so that expenditure in terms of device technology is relatively high.
In view of the above, the invention is based on the object of further developing the generic free-piston engine in such a way as to minimize expenditure in terms of device technology.
This object is achieved through a free-piston engine having the features of claim 1.